1. Field of the Invention
The present invention relates to positron emission tomography (PET) scanners, and more particularly to low-power, low-noise compact PET scanners for use in obtaining an input function from a portion of the human body, such as the wrist.
2. Description of the Prior Art
Positron emission tomography (PET) scanning is a diagnostic tool for non-invasively imaging living organisms. It remains essential to the investigation of chemical and functional processes in biochemistry, biology, physiology, anatomy, molecular biology, and pharmacology. While techniques, such as x-rays, computed tomography (CT), and magnetic resonance imaging (MRI) provide anatomical images, PET scanning provides insight into biochemical changes that generally occur long before a corresponding structural change is detectable by more traditional techniques.
Positrons are positively charged electrons emitted by the nucleus of an unstable radioisotope. The radioisotope is unstable because it is positively charged and has too many protons. Release of the positron stabilizes the radioisotope by converting a proton into a neutron. For radioisotopes used in PET, the element formed from positron decay is stable. All radioisotopes used in PET decay by positron emission. The positron travels a small distance, which depends on its energy, before combining with an electron during a so-called “annihilation event”. The annihilation event ultimately converts the mass of the positron and electron into two gamma rays that are emitted at 180° to each other along a so-called “line of coincidence”. These gamma rays are readily detectable outside the human body.
A small amount of the unstable radioisotope is typically administered to the patient by injection or inhalation, following which it circulates through the body. Scintillation crystals in a tomograph detect the gamma rays emitted by the radioisotope and convert them to light photons. The light photons are then converted to electrical impulses that are processed by the tomograph to determine the location of the annihilation event as being along the line of coincidence.
Kinetic imaging or modeling measures the uptake of a tracer isotope over a period of time. The distribution of tracer isotopes may be used to represent regional blood flow and glucose metabolism. These studies often require catheterization to obtain discrete blood samples, which are analyzed for radioactivity and radioisotope metabolism. Unfortunately, the invasive withdrawal of blood is a significant discomfort to the patient, as well as a significant health risk for both the patient and hospital personnel through exposure to blood borne diseases and radioactive contamination. Therefore, direct arterial blood sampling is considered a health risk for both patients and health workers.
A wide range of quantitative PET studies using tracer kinetic modeling require accurately measured radiotracer concentrations in arterial blood as a function of time after injection, which is commonly referred to as an “arterial input function”. To circumvent the health risks associated with direct arterial blood sampling, several approaches have been examined in an effort to non-invasively obtain an accurate arterial input function. While some approaches have focused primarily on the use of tomography, others have examined additional detector systems that generate a quantitative image-derived input function.
Using the tomograph, studies have examined the possibility of obtaining an input function using large blood vessel imaging. However, this approach is limited in several respects. First, tomography exhibits a partial volume effect defined by spatial resolution. However, an artery large enough to provide reliable data may not be in the field of view. Second, time resolution may be determined by frame acquisition rates specified for a particular study. Although list mode acquisition capabilities reduce restrictions associated with slower acquisition rates, many scanners do not have this capability. Third, subject placement within the tomograph may affect the accuracy of the input function and obtaining reproducible positioning of the body is difficult.
An alternative approach involves placing a radioactivity detector directly over a blood vessel or lung. The primary disadvantage of this approach is the substantial background associated with the surrounding tissue. This background must be subtracted to obtain the true input function. Since this approach is not based on coincidence counts, the signal may include a substantial amount of noise.
Another alternative is to use a standardized input function, which is averaged across many subjects, or a modeled input function. In the latter method, the input function is calculated from various physiological parameters. However, since the input function is very dependent on individual physiological states and procedural variables, such as differences in injection rates, these methods may lead to inaccurate results. Therefore, each of the techniques discussed above yield potential errors and there is a distinct need to determine accurate input functions by measuring the blood activity with little background from remaining portions of the body.
Compact PET detectors require an efficient method of transmitting signals from the detectors to remote electronics for off-line processing. Conventional detectors communicate via independent data links, each of which is dedicated to a particular channel. However, since the majority of PET detectors include hundreds or even thousands of channels, this technique is too cumbersome for a compact PET detector.